Using noninvasive experiments on normal human subjects, this study explores the use of a bolus injection-response technique for quantitating and recognizing mixing patterns at various longitudinal positions in the conducting airways. A device will be developed to automatically inject a series of four identical boli, composed of a 50:50 mixture of helium and krypton, at various points in a quiet 750 ml inspiration of oxygen. During the subsequent expiration the concentrations of the two tracer gases will be continuously monitored by a mass spectrometer while the lung volume at the beginning of inspiration (FRC) and the respiratory flow are also determined. The experiments will be performed at three different respiratory flows with no breathholding and with three different periods of breathholding. Data will be reduced by computing the variance of each of the expired bolus concentration traces. The dispersion coefficient, a parameter proportional to the difference in variance between successively injected boli, indicates the degree of mixing in the longitudinal airway region bounded by the end inspiratory positions of that bolus pair. Thus, the sequential injection of four boli allows the determination of the dispersion coefficients in three interventing airway regions. Moreover, comparison of the dispersion coefficients for the two tracer gases of different molecular weights gives insight into the dominant mixing mechanism in each region. These computations will be performed to determine the longitudinal distribution of mixing for normal respiration as well as during breathholding.